1. Field of the Invention
The invention finds particular utility, for example, in preparing polymeric coating compositions in which pigments, metal powders and flakes, catalysts, silica, mica, charge control agents and other particulate solids are adhered to the surfaces of the compositions. The compositions may be used in fusion coating processes including those which utilize electrostatic deposition. Another suggested field of use of the invention is in the preparation of pharmaceuticals in which solid active ingredients are adhered to particulate polymers, carriers or diluent materials. For ease and clarity of description, the invention is described with particular regard to the formulation of coating powders for use in fusion coating processes although it should be understood that it is not intended that the invention be so limited.
2. Description of the Prior Art
Coating powders are widely used in fusion coating processes in which decorative and protective coatings are applied to metals and other substrates. Because they contain no solvents or volatile organic compounds (VOC's), they are replacing solvent based coatings to prevent the discharge of VOC's to the atmosphere. Coating powders are usually based on thermosetting resin systems comprising a resin, curing agents or another co-reactable resin, additives to control surface tension, gloss, resistance to light and heat, pigments and fillers to control color and reduce cost, and catalysts to reduce the temperature required for curing or to promote faster curing at a higher temperature. A more complete description of coating powders and powder coating processes can be found in the Kirk-Othmer Encyclopedia of Chemical Technology, volume 6, starting on page 635 (1993).
In the early stages of powder coating technology, powders were sometimes mixed by a dry blending process. Here the ingredients were mixed, as in a ribbon blender, ball or impact mill while the powders were maintained at temperatures below their melting point. These processes are no longer used since the processed powders vary in composition from one particle to the next and can not reliably be applied with uniformity of appearance and degree of cure.
Today coating powders are usually made in melt mixing processes in which all the ingredients are thoroughly mixed and dispersed at elevated temperature above the melting point of the base resins, typically in a mixing extruder. The extrudate is cooled, broken into coarse chips and then ground to a fine powder, as in an air impact mill. Typical coating powders have a median particle size of about 30-35 .mu.m and usually have no particles larger than about 180 .mu.m. Powders made by this method are substantially uniform in that each particle contains the starting ingredients in the proportions in which they were initially included in the formulation.
The temperatures at which coating powders are mixed, transported and stored and used in fusion coating processes impose a number of requirements. First, in order to retain the formulated coating powders in a free flowing condition during normal shipping, storage and handling, it is obvious that their softening temperature must be well above anticipated ambient conditions. To insure this result, coating powders should have a glass transition temperature, Tg, of at least about 50.degree. C., and preferably higher.
Opposing this requirement for a moderately high Tg is the need to provide coating powders having low melt viscosities at comparatively low temperatures that provide for satisfactory flow, leveling and cure at the lower temperatures. Coating powders that can be applied at comparatively low temperatures find utility, for example, in the application of clear coats in automotive applications. Typically, a clear coat is applied over plastic or other components that can be harmed at high curing temperatures.
There are other applications where powders which cure at comparatively low temperatures are desired, for example, the coating of heat sensitive articles or assemblies that contain wood, rubber or plastic components. Catalysts are used to increase the rate at which the resin will cure at low temperatures, but they can only be used at relatively low levels. If higher levels are used, the curing reaction may proceed too rapidly in the mixing extruder, leading to an undesirable increase in melt viscosity and loss of flow in the final powder. In extreme cases, excessive cross linking in the extruder will cause gelation of a thermosetting mixture rendering it useless in powder coating processes.
Thermosetting coating powders that are too reactive at low temperatures may react under ambient conditions while being transported or stored. In practical terms, the melt flow of reactive powders may continue to decrease over a matter of days or several months, to the point where they will no longer adequately flow during application. Even currently available commercial powders show a decrease in flow during storage to some degree and the useful life is typically about one year. Very reactive powders, such as those in coating pipe and reinforcing bars (rebar), have a more limited storage life and may require refrigerated shipping and storage. The ability of a powder to retain good melt flow characteristics during storage is frequently referred to as storage stability.
In addition to extending the storage stability of coating powders, it is sometimes desirable to modify their surface characteristics. For example, if a semiconductive coating is needed to bleed static electric charges from sensitive electrical devices, conductive solids like carbon black may be incorporated into the coating powders. The conductive carbon particles are more effective in reducing resistivity if the carbon particles are adhered to the surface of the coating powders rather than being melt mixed in and encapsulated by the insulating coating powders. Similarly, the triboelectric charging characteristics of a powder is improved if a triboelectric additive is adhered to the surface of and not encapsulated in the coating powders since the tribo-electrification of a particle occurs only on its surface.
Aluminum, mica, silica and pigment flakes are used to formulate coating powders that will exhibit a metallic appearance. If these solids are encapsulated in the coating powders, their decorative effectiveness will be concealed. Further, if mixed with the coating powders under conditions of high shear, as in a mixing extruder, the flakes can be ground into such small particles that the metallic effect provided by the solid in flake form is lost and the finish that results from such a coating powder has a dull, gray appearance.
Seemingly the easiest way in which to preserve the identity of the flakes is to dry blend the metal flakes with the coating powder rather than melt mixing them. Unfortunately, the metal flakes may segregate during conventional storage and handling resulting in coatings with irregular and poor surface characteristics. It should also be understood that, in electrostatic coating processes, coating powders are sprayed through a gun and a high voltage corona discharge is used to establish an ionized field. As the powder particles pass through the through the ionized field, they become charged and are attracted to the substrate, which is usually a metallic article to be coated. Subsequently, the powder is fused into a smooth, continuous film. The powder that is not deposited on a substrate is collected in a reclamation system and recycled by mixing the over-spray with virgin coating powders. When metal flakes are loosely mixed with and segregate from the coating powders, the flakes are not charged to the same extent or deposited at the same rate as the coating powders. Over a period of time, the amount of flake in the coating powders will continually increase until a point is reached at which the high concentration of metal flakes interferes with the charging mechanism. If the powder in the ionized field becomes too conductive, the electrostatic gun may "short out". Further, the appearance of the coatings will change as the ratio of flakes to powder changes. It is for these reason that it is important that the solid flake materials be adhered to the surfaces of the coating materials bearing in mind, as discussed above, that it is important that this be accomplished without coating over the surfaces of the flakes as by melt mixing the flakes into the matrices of the coating powders.
For any or all of the above reasons, it is often desirable to mix solid materials with coating powders at temperatures below the melting point of the powders and deposit and attach the flakes to the surface of the powder particles first, so they retain their functional purpose, second, so they properly modify the surface appearance if desired and third, in the case of catalysts, so that they do not prematurely cause the coating powders to begin to cure.
The Prior Patent Art
Several different methods are known to the prior art for variously adhering particulate solids to the surface of powders or impregnating powders with particulate solids.
U.K. Patent Specification 1,404,556 describes coating powders in which aluminum flakes are imbedded into the powder by milling, e.g, in a ball-mill. Since the flakes may be significantly fragmented, the coating powders produced by this method do not achieve the luster of comparable solvent-based metallic paints.
U.S. Pat. No. 3,632,369 discloses a method of mixing dry plastic particles with dry pigments in a high-speed mixer. By maintaining the mixer blades blade at a high angular velocity, the pigment is adheres to the plastic without agglomerating the particles.
U.S. Pat. No. 4,197,351 describes brush polishing a dry mixture of metal flakes and plastic powder so as to embed the flakes into the powder. Although the coating powder obtained may have a high luster, the brush polishing method described in this patent is impractical for industrial scale production of coating powders.
U.S. Pat. No. 4,598,006 is an impregnation process in which active agents are impregnated into pharmaceutical compounds by use of swelling agents.
U.S. Pat. No. 4,820,752 discloses a method of infusing an additive into a polymer by using a compressed fluid that is normally a gas at room temperature. The polymer has some degree of solubility in the compressed gas.
U.S. Pat. No. 5,187,220 adheres a flake material to the surface of coating powders that have been softened by heating the coating powders to close to their melting point. Sufficient mechanical shear is imparted to prevent the agglomeration of the coating powders.
U.S. Pat. No. 5,470,893 discloses a method in which composite particles are formed by the agglomeration of individual particles by mechanical means.